Network communications and control systems for industrial automation (IA) applications historically utilize proprietary protocols transmitted at low data rates over twisted copper pairs. The primary benefits of copper cabling are low cost, ease of termination, and the ability to provide electrical power to remote network devices over the same twisted pair. However, in harsh or hazardous environments where electrical sparks can cause fire, or machines generate high levels of electro-magnetic interference (EMI), or where chemical corrosive of copper wires can occur, etc., optical fibers are often required.
Single-mode and multimode glass optical fibers are high bandwidth and reliable transmission media in wide use from short reach data center server communications to oceanic transcontinental telecommunications. Although optical fiber communication provides significant performance advantages compared to copper media, glass optical fiber and laser transceivers are considerably more expensive and are not easily field terminated compared to their electrical counterparts.
In this disclosure, we describe a hybrid optical and electrical cable assembly utilizing an alternative low-cost Plastic Optical Fiber (POF) media that can support data rates up to 1 Gb/s. The invention provides the benefits of low cost, simple field termination, with the reliable of optical communications with EMI immunity and with the addition of metallic conductors for remote electrical powering.
Plastic Optical Fiber (POF) is a low cost step index core waveguide media made of poly(methyl methacrylate) (PMMA), more commonly known as Plexiglass, Lucite, or Acrylic. Optical transceivers utilize low cost light emitting diodes (LEDs) and silicon photodetectors. Although POF was first developed in the early 1970's, it's in limited use due to its high attenuation and modal dispersion, limiting communications to low data rates over short distances. However, with recent advances in signal processing and advanced modulation schemes such as pulse amplitude modulation (PAM), POF is now capable of supporting 1 Gb/s over a distance of at least 50 m, and 100 Mb/s (Fast Ethernet) up to at least 150 m.
The primary benefit of POF, as well as its primary drawback for optical communications is its large core, 0.96 mm in diameter. The core is surrounded by a thin fluorinated polymer outer cladding layer, giving the fiber an overall diameter of 1 mm. Due to the large core of POF (in comparison to 9-microns for single-mode and 50-microns for multimode glass optical fiber, POF supports thousands of fiber modes and consequently, has high modal dispersion and low signal bandwidth. However, the large core also relaxes alignment tolerances required for mechanical termination and alignment to transceiver components. Unlike glass fiber termination, POF does not require split sleeves or any other precision component for fiber-to-fiber core alignment and does not requiring end face polishing. Therefore, although POF has limited bandwidth, the termination cost is extremely low, typically less than copper for data rates of 100 Mb/s and 1 Gb/s.
For typical industrial applications, the cabling and terminations require ingress protection against the intrusion of solid objects and/or water. The degree of protection is classified by the International Protection (IP) ratings (also referred to as Ingress Protection ratings) and is specified in the international standard IEC 60529. The IP code consists of the letters IP followed by two digits and an optional letter, where the first digit specifies the degree of protection against solid foreign objects, and the second against water.
The most common styles of electrical and optical cable terminations used for factory automation, process control, industrial instrumentation, and commercial electronics are the M12 and M8 connectors, which are circular threaded screw-lock connectors as shown in FIG. 1. For optical fiber terminations the M8 or M12 inner housings, which typically contain the electrical contacts are replaced with standardized optical connectors and adapters specified in the Fiber Optical Connector Intermateability Standard (FOCIS), most commonly the LC push pull type connector shown in FIG. 2.
In this invention, the optical fibers and electrical pins are interchangeable within the same connector. In one embodiment of the present invention, two electrical contact pins in a 4-pin M8 style connector are removed and replaced with two POF fibers, eliminating the need for costly FOCIS compliant connectors and adapters. The elimination of optical connectors and adapters is possible because of the low alignment tolerance required for POF interconnections, and the fact that the POF has the same lmm outside diameter as the typical electrical pin contacts. By replacing all but two of electrical contacts, the connector can be adapted to accept a duplex pair of POF, while the remaining two electrical contacts provide power and ground for remote powering. To secure the POF fibers and provide adequate end face physical contact, some modifications to the M8 (or M12) connector assemblies are required as exemplified in the following sections. The larger M12 connector enables additional POF pairs and/or electrical contacts to be installed.